Treatment of organic dielectric materials with high-frequency electric current



Patented Dec. 12, 1944 TREATMENT OF ORGANIC DIELECTRIC l VIA- TERIALS"WITH HIGH-FREQUENCY ELEC- TRIC CURRENT I Charles B. Hemming, Parlln, N.J assignor to E. I. du Pont de Nemours & Company, Wilmington,

Del., a corporation of Delaware llo Drawing. Application September 13,1941, Serial No. 410,785

7 Claims.

This invention relates to the application of high frequencyelectrostatic fields to organic substantially non-conducting materialsand more particularly to a means for increasing the electrical losses ofsuch materials when exposed to high frequency electrostatic fields.

external heat sources.

tions.

objects.

mercial practice.

absorbed by metallic conductors.

stants of 20 or less at 60 cycles.

times the power factor. The larger the product of these constants, thegreater is the energy absorption from a radio frequency field. Forexample, polystyrene with a dielectric constant of 2.5 and a powerfactor of 0.0001 (at 60 cycles) Methyl A further object is the The heat-The high dielectric conhas as its product a value of 0.00025.

A unique method for joining a plurality of methacrylate withadielectricconstant of 3.5 and parts by exposing a thermo-sensitive adhesive apower factor of 0.06 has as its product the value having relatively highhysteresis characteristics 0.21 or one over 800 times as great asstyrene. to high frequency electrostatic field is described 10 In anelectrostatic field having a frequ y of in U. S. Patent 2,087,480. Itwas found that 30 megacycles per second and animpressed voltsufilcientheating occurs to soften or fuse or a e f 2 00. m t yl t y t heatsreadily otherwise affect the adhesive to cause bonding. wh l p y tyr spractically unaffected- It Previously heat had been applied through consi po ant s a r ul f t fact to develop duction by hot pressure plates,ovens or oth means for modifying the more perfect dielectrics Dependentupon the heat so that they will heat readily and quickly in theconductivity of the parts to be joined, the time radio frequency fields.necessary to secure satisfactory joints was often It is an object ofthis invention to P o ide a long and seriously delayed manufacturingoperameans of controlling the absorption of electrical As suggested inthe patent, it is characen y y organic p m ri ma perticw teristic of theaction of high frequency fields that y thermoplastic. Organic resinousdielectrics uniform and rapid heating of the materials ext radio f qposed occurs, that is, absorption of electrical pr vision f a m n of inre s he energy energy occurs throughout the materials, limited absorptiy the more p f t d tr A y by the hysteresis characteristics of the stillfurther object is the provision of modifiers terials exposed. Excessivesurface heating is, for organic po y c materials which (10 nottherefore, avoided, and the time required to heat t y alter h physicalproperties of the thick objects is substantially the same as for thin poi io O r objects w l app h r I have found that titanium dioxide in itsrutile It has been found advantageous to use elecand br k e rystallineforms has electrical trical frequencies in the more commonly used p p tSuch t although t p ave y radio frequency range of 100,000 cycles to 100cflicient dielectric alone at radio frequencies, it megacycles persecond. The oltage applied may acts in an unexpected manner whenincorporated be varied over wide ranges but in general an insubstantially non-conducting organic c p input voltage of at least 1000is necessary in comi io and giv ed mp n f greater hysteresischaracteristics than represented by the The process is particularlyadaptable to the titanium d e f ate cementing of the component parts ofshoes, for Titanium dioxide is known in three crystalline themanufacture of plywood, for cementing cork forms: rutile, brookite, andanatase. granules and many other cementing operations. 40 .ing ofbrookite and anatase above 820 0. tends The process is particularlyadvantageous where toconvertthem totherutileform. Allthreeforms thematerials to be joined are non-metallic and possess high dielectricconstants, with those of are poor conductors of heat. It is acharacterrutile and brookite having the very abnormal istic of theelectrostatic field that in the radio dielectric constants ofapproximately 114 and 78, frequency range very little electrical energyis respectively. The anatase form shows a dielec- The system is, tricconstant of 3|. therefore, adapted to organic polymeric matestants arecoupled with very low power factors, rials which are ordinarilyconsidered to be good so that the product of the dielectric constantdielectrics for direct or low frequency alternating times the powerfactor is low. The tendency of currents, that is, materials havingdielectric conthe rutile to absorb electrical energy from a radiofrequency field is slight, and the material can be It is generallyassumed that the extent of the used as an insulator in these fields.hysteresis loss of a material can be expressed Unexpectedly, however,when .finely divided comparative to those of other materials by comrutile or brookite is incorporated into a polyparing the products of thedielectric constant meric organic material, the hysteresis losses ofobtained. In many cases adhesives which cannot be made to heatsatisfactorily in the available electrostatic fields can be modifiedwith the rutile or brookite to produce materials which can be activated.

The following examples are given by way of iilustration and nolimitations are intended thereby except as indicated in the presentinvention:

Example 1 Percent by 1 weight Polyvinyl butyral resin; 1.8.7 Dibutylphthalate g 9.4 Denatured ethyl alcohol 36.3 Tnlunl 35.6

This example was-comparedin heating characteristics in a radio frequencyfield of 2000 Volts at megacycles per second, with the same compositionin which 34% by weight of the total solids content of finely dividedrutile had been incorporated by rapid mixing. Both compositions wereapplied by brushing to the rough side of 1 x 6 inch strips of shoe soleleather and allowed to dry hour at 60 C.

A second coat was then applied and dried for ,5 hour at 60 C. followingwhich they were allowed to stand for 4 hours in a room at 245 C.

Heating tests were then carried out using a treater input voltage of2100 at a frequency of 30 megacycles. between two adjustable metalplates, between which the leather samples were placed in the same plane.leather strips were placed between the treater plates for seconds. Thetemperature of the strips at the end of the treatment was obtained byinserting a quick-acting metallic thermometer of the Weston type betweenthe strips before any appreciable heat could be dissipated through theleather. The strips coated with the unmodifled adhesive composition weretested under the same electrical conditions and finally the stripscoated with the adhesive containing the rutile were similarly tested.The following averaged results were obtained:

1 Rise above m room temperature 36', C. Leather stri -no adhesivm 65 40Leather ad esive 71 46 Leather adhesive-Hume-.. 76 50 Longer periods oftreatment gave higher temperature rises but 'of a comparable order ofdifference. Excellent bonds were secured with the adhesive containingthe rutile modifier.

The electrostatic field was set upv As the initial test, twouncoatedsame 2 Percent by weight Ethyl cellulose 18.7 Dibutyl phthalate9.4 Ethyl alcohol (denatured) 38.2 Toluol- 35.7

66.2 parts by weight of this composition were mixed with 33.8 parts byweight of rutile and the mixture applied by brushing two coats on four1'' x 6" sole leather strips. Similarly, two coats of the unmodifiedcomposition were applied to four other leather strips. All samples weredried at C. for /2 hour between coats and subjected to a similar dryingperiod after the second coat and then cooled for 3 hours at atemperature of 24.5 C. and a relative humidity of 31%.

Pairs of uncoated and coated leather strips with the coated surfacesheld together under moderate pressure with flexible rubber bands weresuspended in a high frequency electrostatic field of 30 megacycles persecond and an applied input voltage to the oscillator of 2100 volts fora period of 45 seconds and the rise in temperature determined as inExample 1.

The following averaged results were obtained:

Further illustrative of the novel and desirable effects afforded by theuse of rutile for inducing heat in organic dielectric materials are thepower absorption results obtained by testing sheets of polymerizedmethyl methacrylate. Two sheets of this material 1 x 3" and /a" inthickness and held together with flexible rubber bands were suspended ina high frequency field of 30.6 megacycles per second and an impressedinput voltage to the oscillator of 2100 volts. Power absorption wasfound to be 21 watts. This test was repeated with a layer of rutileamounting to about 50% of the weight of the sheets uniformly interposedbetween the two sheets of polymerized methyl methacrylate resin and inthis case the power absorption was 63 watts.

-As indicated above, titanium dioxide in the rutile or brookite form ispreferred in the operation of the present invention because of theirunusually highdielectric constants. The anatase form with its relativelylow dielectric constant fails to provide the advantages of the rutileand brookite forms and, therefore, is of little use as a means forincreasing the hysteresis losses in the polymeric organic materialsherein described.

It has been found that an amount of rutile or brookite of between about20 to 50% by weight of the total solids of the composition to be treatedgives improved results with about 30 to 40% by weight of the totalsolids giving greatest improvement.

The invention is of general utility for inducing heat in organicpolymeric materials when such materials are exposed to energizedelectrostatic fields of radio frequencies, preferably frequenciesbetween about 100,000 cycles and megacycles and especially atfrequencies of merized state including phenolic, urea aldehyde.

acrylic acid resins, alkyd and styrene type resins promotes the heatingof such materials when exposed to high frequency electrostatic fieldsthereby converting them to the polymeric state. The agents are alsouseful in cellulose derivative compositions containing cellulosenitrate, cellulose acetate, cellulose ethers and mixed cellulose esterssuch as cellulose acetobutyrate and cellulose acetopropionate.

- Of particular advantage is the increase in hysteresis losses at highfrequencies occasioned by the addition of the rutile or brookite formsof titanium dioxide to organic polymeric dielectric materials. In someinstances, such materials have such low losses at radio frequencies thatno heating results are experienced when they are placed in anelectrostatic field of such fre quencies. However, the incorporation ofthe suggested modifying agents, even though they are excellentdielectric materials themselves. causes a change in the electricalproperties of the organic material so that the hysteresis losses areincreased to a point that heating results at such frequencies. Theinvention presents desirable advantages even with organic polymericmaterials which normally respond to electrostatic fields of frequenciesbetween 100,000 cycles and 100 megacycles in that the rate and amount ofheat so induced is markedly increased.

It is apparent that many widely different embodiments of the inventionmay be made without departing from the spirit and scope thereof and,therefore, it is not intended to be limited except as indicated in theappended claims.

'I claim:

1. The process of inducing heat in organic materials of high dielectricvalue which coniprises dispersing titanium dioxide selected from thegroup consisting of rutile and brookite in said organic material andplacing the so modified material in an energized electrostatic field ofradio frequency.

2. Process of claim 1 in which the frequency of the electrostatic fieldis between 100,000 cycles per second and 100 megacycles per second.

3. Process of claim 1 in which the frequency of the electrostatic fieldis about 30 megacycles per second.

4. In the process of heating organic monomeric and polymeric materialsof high dielectric value by exposing said materials to an energized highfrequency electrostatic field, the step of incorporating titaniumdioxide selected from the group consisting of rutile and brookite insaid organic material and thereafter placing the so modified material inan energized high frequency field, said frequency being from 100,000cycles per second to 100 megacycles per second.

5. Process of claim 4 in which the titanium dioxide is present in anamount between about 20 and by weight of the total solids.

6. Process. for promoting the polymerization of unpolymerized andmonomeric organic materials of high dielectric value which comprisesdispersing titanium dioxide selected from the group consisting of rutileand brookite in said organic material and exposing the so modifiedmaterial to an energized electrostatic high frequency field until thematerial is polymerized. said frequency being from 100,000 cycles persecond to megacycles per second.

7. Process of claim 6 in which the monomeric material is monomericstyrene resin.

CHARLES B. HEMMING.

